Electrophoretic core-shell particles having an organic pigment core and a shell with a thin metal oxide layer and a silane layer

ABSTRACT

An electrophoretic medium comprises a plurality of core-shell particles and a non-polar fluid. The core-shell particles comprise an organic pigment particles core and a shell comprising a metal oxide layer and a silane layer. The metal oxide layer may have a thickness of 0.4 to 2 nm. It may be formed using a fluidized bed reactor by inserting the organic pigment into the reactor as a powder bed, contacting the powder bed with a gaseous stream comprising a metal oxide precursor and an inert gas, and contacting the powder bed with a gaseous stream of a reagent and an inert gas. The silane layer is formed from a silane compound comprising a first functional group, wherein the first functional group reacts with the metal oxide.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of the U.S.Provisional Application having Ser. No. 62/970,901 filed on Feb. 6,2020, the content of which is incorporated by reference herein in itsentirety.

FIELD OF THE INVENTION

This invention relates to organic pigments used in electrophoreticdisplay media. More specifically, in one aspect this invention relatesto electrophoretic systems containing core-shell organic pigmentparticles having a shell comprising a thin metal oxide layer and asilane layer.

BACKGROUND OF INVENTION

This invention relates to particles for use in electrophoretic displays,and to electrophoretic media and displays incorporating such particles.More specifically, the invention relates to an electrophoretic mediumcomprising a plurality of core-shell particles and a non-polar liquid.In one aspect, the core-shell particles comprise (a) a core comprisingan organic pigment and (b) a shell comprising a metal oxide layer and asilane layer, wherein the metal oxide layer is formed by deposition of ametal oxide onto the organic pigment surface using a fluidized bedreactor.

The term “electro-optic”, as applied to a material or a display, is usedherein in its conventional meaning in the imaging art to refer to amaterial having first and second display states differing in at leastone optical property, the material being changed from its first to itssecond display state by application of an electric field to thematerial. Although the optical property is typically color perceptibleto the human eye, it may be another optical property, such as opticaltransmission, reflectance, luminescence or, in the case of displaysintended for machine reading, pseudo-color in the sense of a change inreflectance of electromagnetic wavelengths outside the visible range.

Some electro-optic materials are solid in the sense that the materialshave solid external surfaces, although the materials may, and often do,have internal liquid- or gas-filled spaces. Such displays using solidelectro-optic materials may hereinafter for convenience be referred toas “solid electro-optic displays”. Thus, the term “solid electro-opticdisplays” includes rotating bichromal member displays, encapsulatedelectrophoretic displays, microcell electrophoretic displays andencapsulated liquid crystal displays.

The terms “bistable” and “bistability” are used herein in theirconventional meaning in the art to refer to displays comprising displayelements having first and second display states differing in at leastone optical property, and such that after any given element has beendriven, by means of an addressing pulse of finite duration, to assumeeither its first or second display state, after the addressing pulse hasterminated, that state will persist for at least several times, forexample at least four times, the minimum duration of the addressingpulse required to change the state of the display element. It is shownin U.S. Pat. No. 7,170,670 that some particle-based electrophoreticdisplays capable of gray scale are stable not only in their extremeblack and white states but also in their intermediate gray states, andthe same is true of some other types of electro-optic displays. Thistype of display is properly called “multi-stable” rather than bistable,although for convenience the term “bistable” may be used herein to coverboth bistable and multi-stable displays.

One type of electro-optic display, which has been the subject of intenseresearch and development for a number of years, is the particle-basedelectrophoretic display, in which a plurality of charged particles movethrough a fluid under the influence of an electric field.Electrophoretic displays can have attributes of good brightness andcontrast, wide viewing angles, state bistability, and low powerconsumption when compared with liquid crystal displays. Nevertheless,problems with the long-term image quality of these displays haveprevented their widespread usage. For example, particles that make upelectrophoretic displays tend to settle, resulting in inadequateservice-life for these displays.

As noted above, electrophoretic media require the presence of a fluid.In most prior art electrophoretic media, this fluid is a liquid, butelectrophoretic media can be produced using gaseous fluids; see, forexample, Kitamura, T., et al., “Electrical toner movement for electronicpaper-like display”, IDW Japan, 2001, Paper HCS1-1, and Yamaguchi, Y.,et al., “Toner display using insulative particles chargedtriboelectrically”, IDW Japan, 2001, Paper AMD4-4). See also U.S. Pat.Nos. 7,321,459 and 7,236,291. Such gas-based electrophoretic mediaappear to be susceptible to the same types of problems due to particlesettling as liquid-based electrophoretic media, when the media are usedin an orientation which permits such settling, for example in a signwhere the medium is disposed in a vertical plane. Indeed, particlesettling appears to be a more serious problem in gas-basedelectrophoretic media than in liquid-based ones, since the lowerviscosity of gaseous suspending fluids as compared with liquid onesallows more rapid settling of the electrophoretic particles.

Numerous patents and applications assigned to or in the names of theMassachusetts Institute of Technology (MIT), E Ink Corporation, E InkCalifornia, LLC. and related companies describe various technologiesused in encapsulated and microcell electrophoretic and otherelectro-optic media. Encapsulated electrophoretic media comprisenumerous small capsules, each of which itself comprises an internalphase containing electrophoretically-mobile particles in a fluid medium,and a capsule wall surrounding the internal phase. Typically, thecapsules are themselves held within a polymeric binder to form acoherent layer positioned between two electrodes. In a microcellelectrophoretic display, the charged particles and the fluid are notencapsulated within microcapsules but instead are retained within aplurality of cavities formed within a carrier medium, typically apolymeric film. The technologies described in these patents andapplications include:

-   -   (a) Electrophoretic particles, fluids and fluid additives; see        for example U.S. Pat. Nos. 6,822,782; 7,002,728; 7,679,814;        8,018,640; 8,199,395; and 9,372,380; and U.S. Patent Application        Publication No. US2018/0210312;    -   (b) Capsules, binders and encapsulation processes; see for        example U.S. Pat. Nos. 6,922,276; and 7,411,719;    -   (c) Microcell structures, wall materials, and methods of forming        microcells; see for example U.S. Pat. Nos. 7,072,095; and        9,279,906;    -   (d) Methods for filling and sealing microcells; see for example        U.S. Pat. Nos. 7,144,942; and 7,715,088;    -   (e) Films and sub-assemblies containing electro-optic materials;        see for example U.S. Pat. Nos. 6,982,178; and 7,839,564;    -   (f) Backplanes, adhesive layers and other auxiliary layers and        methods used in displays; see for example U.S. Pat. Nos.        7,116,318; 7,535,624;    -   (g) Color formation and color adjustment; see for example U.S.        Pat. Nos. 7,075,502 and 7,839,564;    -   (h) Methods for driving displays; see for example U.S. Pat. Nos.        7,012,600; and 7,453,445;    -   (i) Applications of displays; see for example U.S. Pat. Nos.        7,312,784; 8,009,348;    -   (j) Non-electrophoretic displays, as described in U.S. Pat. Nos.        6,241,921; and 2015/0277160; and applications of encapsulation        and microcell technology other than displays; see for example        U.S. Patent Application Publications Nos. 2015/0005720 and        2016/0012710.

Many of the aforementioned patents and applications recognize that thewalls surrounding the discrete microcapsules in an encapsulatedelectrophoretic medium could be replaced by a continuous phase, thusproducing a so-called polymer-dispersed electrophoretic display, inwhich the electrophoretic medium comprises a plurality of discretedroplets of an electrophoretic fluid and a continuous phase of apolymeric material, and that the discrete droplets of electrophoreticfluid within such a polymer-dispersed electrophoretic display may beregarded as capsules or microcapsules even though no discrete capsulemembrane is associated with each individual droplet; see for example,the aforementioned U.S. Pat. No. 6,866,760. Accordingly, for purposes ofthe present application, such polymer-dispersed electrophoretic mediaare regarded as sub-species of encapsulated electrophoretic media.

Although electrophoretic media are often opaque (since, for example, inmany electrophoretic media, the particles substantially blocktransmission of visible light through the display) and operate in areflective mode, many electrophoretic displays can be made to operate ina so-called “shutter mode” in which one display state is substantiallyopaque and one is light-transmissive. See, for example, U.S. Pat. Nos.5,872,552; 6,130,774; 6,144,361; 6,172,798; 6,271,823; 6,225,971; and6,184,856. Dielectrophoretic displays, which are similar toelectrophoretic displays but rely upon variations in electric fieldstrength, can operate in a similar mode; see U.S. Pat. No. 4,418,346.Other types of electro-optic displays may also be capable of operatingin shutter mode. Electro-optic media operating in shutter mode may beuseful in multi-layer structures for full color displays; in suchstructures, at least one layer adjacent the viewing surface of thedisplay operates in shutter mode to expose or conceal a second layermore distant from the viewing surface.

An encapsulated electrophoretic display typically does not suffer fromthe clustering and settling failure mode of traditional electrophoreticdevices and provides further advantages, such as the ability to print orcoat the display on a wide variety of flexible and rigid substrates.(Use of the word “printing” is intended to include all forms of printingand coating, including, but without limitation: pre-metered coatingssuch as patch die coating, slot or extrusion coating, slide or cascadecoating, curtain coating; roll coating such as knife over roll coating,forward and reverse roll coating; gravure coating; dip coating; spraycoating; meniscus coating; spin coating; brush coating; air knifecoating; silk screen printing processes; electrostatic printingprocesses; thermal printing processes; ink jet printing processes;electrophoretic deposition (See U.S. Pat. No. 7,339,715); and othersimilar techniques.) Thus, the resulting display can be flexible.Further, because the display medium can be printed (using a variety ofmethods), the display itself can be made inexpensively.

However, the service life of encapsulated electrophoretic displays isstill lower than is altogether desirable. It appears that this servicelife is limited by factors such as the tendency of particles toaggregate into clusters, which prevent the particles completing themovements necessary for switching of the display between its opticalstates. The physical properties and surface characteristics ofelectrophoretic particles can be modified by adsorbing various materialsonto the surfaces of the particles, or chemically bonding variousmaterials to these surfaces. For example, in an electrophoretic displaythat contains organic pigments, monomers having different chemicalgroups may form polymer coatings on the pigments by dispersionpolymerization and the coatings may interact with a charge control agentto provide colored particles of varying charge strength. An analogousapproach for achieving improved electro-optic performance is the use oforganic pigment particles coated with a metal oxide, such as silica. Thesilica coating enables the covalent attachment of other materials on thepigment particle surface, creating stable, differentiated surfaces amongthe different types of electrophoretic particles in a medium. A typicalmethod of silica precipitation onto organic pigment particles involvesthe reaction of hydrolysable silicone materials, such astriethoxysilane, with water in an organic solvent and in the presence ofthe organic pigment particles. Absent a very tight control of theconditions of the process, the coating is not uniform with areas of nocoverage and areas of a thicker silica coating. This may lead to lessefficient particle separation and to a reduction of the color propertiesof the organic pigment. Thus, there is a need for improvedelectrophoretic particles and processes of making thereof.

SUMMARY OF INVENTION

According to one aspect of the present invention, an electrophoreticmedium comprises a plurality of a first type of core-shell particles anda non-polar fluid. Each of the plurality of the first type of thecore-shell particles comprises a core, comprising an organic pigment,and a shell. The shell comprises a metal oxide layer and a silane layer.The thickness of the metal oxide layer is from about 0.4 nm to about 2nm. The silane layer is formed from a silane compound comprising a firstfunctional group, wherein the first functional group reacts with themetal oxide. The electrophoretic medium may further comprise a pluralityof a second type of core-shell particles or another type of chargedelectrophoretic particles. The electrophoretic medium may also comprisemore than two types of core-shell particles or other types of chargedelectrophoretic particles. This electrophoretic medium can be used in anelectrophoretic device that comprises a first light-transmissiveelectrode layer, an electro-optic material layer, and a second electrodelayer. The electro-optic material layer of the electrophoretic devicecomprises encapsulated electrophoretic medium, that is, microcapsules ormicrocells containing particles in a fluid. The electrophoretic mediumcan also be used in electrophoretic assemblies such as front planelaminates, inverted front plane laminates, and double release sheets.

According to another aspect of the present invention, an electrophoreticmedium comprises a plurality of a first type of core-shell particles anda non-polar fluid. Each of the plurality of the first type of thecore-shell particles comprises a core, comprising an organic pigment,and a shell. The shell comprises a metal oxide layer and a silane layer.The metal oxide layer is formed on the surface of the organic pigmentusing a fluidized bed reactor by inserting the organic pigment into thereactor as a powder bed, contacting the powder bed with a gaseous streamcomprising an inert gas and a metal oxide precursor, and contacting thepowder bed with a gaseous stream of an inert gas and a reagent thatreacts with the metal oxide precursor to form a metal oxide. Theelectrophoretic medium may further comprise a plurality of a second typeof core-shell particles or another type of charged electrophoreticparticles. The electrophoretic medium may also comprise more than twotypes of core-shell particles or other types of charged electrophoreticparticles. This electrophoretic medium can be used in an electrophoreticdevice that comprises a first light-transmissive electrode layer, anelectro-optic material layer, and a second electrode layer. Theelectro-optic material layer of the electrophoretic device comprisesencapsulated electrophoretic medium, that is, microcapsules ormicrocells containing particles in a fluid. The electrophoretic mediumcan also be used in electrophoretic assemblies such as front planelaminates, inverted front plane laminates, and double release sheets.

According to another aspect of the present invention, the shell of thecore-shell particles further comprises a polymer stabilizer layer. Thepolymer stabilizer layer may be formed from the reaction of the silanelayer and a monomer or macromonomer, wherein the silane compound usedfor the formation of the silane layer comprises a third functional groupand the monomer or macromonomer comprises a fourth functional group. Thepolymer stabilizer layer is formed by the reaction between the third andfourth functional groups.

According to another aspect of the present invention, a method ofmanufacturing of an electrophoretic medium comprising a plurality ofcore-shell particle and a non-polar fluid comprising the steps of (a)providing organic pigment particles; (b) introducing the organic pigmentparticles into a fluidized bed reactor as a powder bed; (c) contactingthe powder bed with a gaseous stream comprising an inert gas and aprecursor of metal oxide; (d) contacting the powder bed with a gaseousstream comprising a reagent to form organic pigment particles having ametal oxide layer on its surface, wherein the reagent is selected fromthe group consisting of water, oxygen, ozone, and mixtures thereof; (e)reacting the organic pigment particles having a metal oxide layer with asilane compound in an organic solvent to form a silane layer, whereinthe silane compound comprises a first functional group and a thirdfunctional group, wherein the first functional group reacts with themetal oxide to form organic pigment particles comprising a metal oxidelayer and a silane layer; and (f) combining the plurality of core-shellparticles and the non-polar fluid. The gaseous stream comprising thereagent may further comprise an inert gas.

BRIEF DESCRIPTION OF THE FIGURES

Various aspects and embodiments of the application will be describedwith reference to the following figures. It should be appreciated thatthe figures are not necessarily drawn to scale.

FIG. 1A shows the reaction scheme for the formation of a core-shellparticle comprising an organic pigment core and a shell comprising ametal oxide layer. FIG. 1B shows the reaction scheme for the formationof a core-shell particle comprising an organic pigment particle core anda shell comprising a metal oxide layer and a silane layer. FIG. 1C showsthe reaction scheme for the formation of a core-shell particlecomprising an organic pigment core and a shell comprising a metal oxidelayer, a silane layer, and a polymer stabilizer layer.

FIG. 2 is a schematic illustration of an electrophoretic devicecomprising an electro-optic material layer comprising encapsulatedelectrophoretic medium;

FIG. 3 is a schematic illustration of an electro-optic assembly that isa front plane laminate comprising a first light-transmissive layer, anelectro-optic material layer comprising encapsulated electrophoreticmedium, an adhesive layer, and a release sheet.

FIG. 4 is a schematic illustration of an electro-optic assembly that isa double release sheet comprising a first release sheet, a firstadhesive layer, an electro-optic material layer comprising encapsulatedelectrophoretic medium, a second adhesive layer, and a second releasesheet.

FIG. 5 shows the reaction scheme for the formation of a core-shellparticle comprising an organic pigment particle and a shell comprising ametal oxide layer, a silane layer, and a polymer stabilizer layer.

DETAILED DESCRIPTION

The present invention provides an electrophoretic medium comprising aplurality of a plurality of core-shell particles and a non-polar fluid.The core-shell particles comprise an organic pigment core and a shellcomprising a metal oxide layer and a silane layer. The shell may alsocomprise a polymer stabilizer layer. This layer, if it is present, islocated at the surface of the core-shell particle and it contributes tothe dispersion stability of the particles in the electrophoretic medium.

The electrophoretic medium of the present invention may be incorporatedinto an electro-optic display. A typical electro-optic display,comprising an electrophoretic medium, also comprises a first (front)electrode and a second (rear) electrode. The first electrode islight-transmissive. The second electrode may be also light-transmissive,or it may be not light-transmissive. The electrophoretic mediumcomprising a plurality of charged particles in a non-polar fluid istypically positioned between the front and rear electrodes.

The electrophoretic medium may contain one or more types of particles,which may have different color and charges. For example, there arecommercial electro-optic displays that comprise electrophoretic mediumhaving oppositely charged white and black particles. However, displayscomprising one or more types of charged organic particles also exist inthe market. The use of organic pigments is preferred because theyprovide brighter and more saturated color in comparison to inorganicpigments. Typical organic pigments used in electro-optic displays mayhave cyan, magenta, yellow, red, green, blue, and black color.Non-limiting examples of organic pigment types include azo,phthalocyanine, quinacridone, perylene, diketopyrrolopyrrole,benzimidazolone, isoindoline, anthranone, indanthrone, rhodamine,benzinamine, and carbon black types. Although many practitionersconsider carbon black pigment as an inorganic pigment, this type ofpigment is considered an organic pigment for the purpose of the presentpatent application, as some of its physical characteristics, such ashydrophobicity, surface area, etc., resemble those of an organicpigment. Non-limiting examples of specific organic pigments that may beused in electrophoretic media include C.I. Pigment Blue 15, 15:1, 15:2,15:3, 15:4 15:6, 60, and 79; Pigment Red 2, 4, 5, 9, 12, 14, 38, 48:2,48:3, 48:4, 52:2, 53:1, 57:1, 81, 112, 122, 144, 146, 147, 149, 168,170, 176, 177, 179, 184, 185, 187, 188, 208, 209, 210, 214, 242, 254,255, 257, 262, 264, 282, and 285; C.I. Pigment Violet 1, 19, 23, and 32;C.I. Pigment Yellow 1, 3, 12, 13, 14, 15, 16, 17, 73, 74, 81, 83, 97,109, 110, 111, 120, 126, 127, 137, 138, 139, 150, 151, 154, 155, 174,175, 176, 180, 181, 184, 191, 194, 213 and 214; C.I. Pigment Green 7,and 36; C.I. Pigment Black 1, and 7; C.I. Pigment Brown 25, 32, 41;Pigment Orange 5, 13, 34, 36, 38, 43, 61, 62, 64, 68, 67, 72, 73, and74.

The organic pigment of the core of the core-shell particles of thepresent invention may have average diameter from 1 nm up to about 100μm, or from 50 nm to 1 μm, or from 60 nm to 800 nm.

Organic pigment provide color because they absorb specific wavelengthsof incident light that correspond to visible light. Typically, theircolor saturation and strength increases with decreasing particle size(that is, with increasing surface area). Thus, they are mostly availableas relatively high surface area particles, which make them relativelydifficult to disperse and stabilize in liquid carriers.

According to one embodiment of the present invention, an electrophoreticmedium comprises a plurality of a first type of core-shell particles anda non-polar fluid. The core comprises an organic pigment, and the shellcomprises a metal oxide layer and a silane layer.

The metal oxide layer is formed on the surface of the organic pigmentusing a fluidized bed reactor by inserting the organic pigment into thereactor as a powder bed, contacting the powder bed with a gaseous streamcomprising an inert gas and a metal oxide precursor, and contacting thepowder bed with a gaseous stream of an inert gas and a reagent. Thereagent reacts with the metal oxide precursor to form a metal oxide.

A continuous or a batch process may be used in the fluidized bedreactor. A typical fluidized bed reactor comprises a chamber that enablethe intimate mixing of powder materials with gaseous phase reagents inorder to improve both the reaction rate and the uniformity of themodified powder surfaces. The pigment powder can be placed on a porousplate as a powder bed inside the reactor and positioned vertically. Agaseous stream of the metal oxide precursor/inert gas can be injectedfrom an inlet at the top or at the bottom of the chamber of the reactor.For example, a gaseous stream of trimethyl aluminum in nitrogen gas maybe used. The bed may be continuously vibrated. In addition, the gaspressure may drop because of the resistance from the powder bed, whichmay also cause continuous movement of the powder and suspension of thepowder in the gas stream, improving the contact between the surface ofthe powder and the reagent. The metal oxide precursor may be complexed(or adsorbed) on the surface of the powder in a thin layer. The gaseousstream may then be changed from the metal oxide precursor to areagent/inert gas stream, for example water/nitrogen. This gaseousstream will cause the reaction of the thin film of the metal oxideprecursor with the reagent, forming a thin layer on the surface of thepowder. In a continuous process using a horizontal fluidized bedreactor, the powder bed may be continuously transported from one side ofthe chamber to the other, passing through the separate zones, where thepowder is exposed to separate gaseous streams (the metal oxideprecursor/nitrogen stream and then the reagent/nitrogen stream). It ispossible to include a separate stream before the delivery of thesurface-treated powder from the outlet of the fluid bed reactor, whichmay cause the removal of the excess reagent, for example, the drying ofthe pigment powder. An example of a continuous process is provided inU.S. Patent Application No. US2018/0363136 by ALD Nanosolutions, Inc.

FIG. 1A illustrates the reaction scheme by which the metal oxide layeris formed on the surface of an organic pigment particle 101. A gaseousstream of a metal oxide precursor in an inert gas is brought intocontact with a powder bed of organic pigment particles 101. Theprecursor is complexed or adsorbed on the surface of the organic pigmentparticle and it reacts with a subsequent gaseous steam of a reagent inan inert gas to form a metal oxide layer 102, having a layer thicknessof L1. Particle 102 (organic pigment particle having a metal oxide layeron its surface) is then reacted with a silane compound 103 as shown inFIG. 1B. This silane has a substituent F1 comprising a first functionalgroup, which can react with the metal oxide surface to provide acore-shell particle 104. The core-shell particle 104 has a metal oxidelayer with a thickness L1 and a silane layer with a thickness of L2. Thesilicon atom of the silane compound also has substituents R1, R2, andR3. One or two of these substituents may also be substituent F1,comprising a first functional group, which may also react with the metaloxide surface. At least one of R1, R2, and R2 may comprise a secondfunctional group that may provide a charge to the particle or it canmodify a surface characteristic of the core-shell particle such as itssurface energy.

The metal oxide layer may comprise aluminum oxide, silica, titaniumdioxide, zirconium oxide, zinc oxide or mixtures thereof.

Non-limiting examples of a metal oxide precursors are trimethylaluminum,triethylaluminum, dimethylaluminum chloride, diethylaluminum chloride,trimethoxyaluminum, triethoxyaluminum, dimethylaluminum propoxide,aluminum triisopopoxide, tributoxy aluminum, tris(dimethylamino)aluminum, tris(diethylamino) aluminum, tris(propylamino) aluminum,aluminum trichloride, trichlorosilane, hexachlorodisilane, silicontetrachloride, tetramethoxysilane, tetraethoxysilane,tris(tert-pentoxy)silanol, tetraisocyanatesilane, silicon tertrachoride,tris(methylamino)silane, tris(ethylamino)silane, titanium tetrachloride,titanium tetraiodide, tetramethoxy titanium, tetraethoxy titanium,titanium isopropoxide, tetrakis(methylamino) titanium,tetrakis(ethylamino) titanium, dimethyl zinc, diethyl zinc, methyl zincisopropoxide, zirconium tetrachloride, zirconium tetraiodide,tetramethoxy zirconium, tetraethoxy zirconium, tetraisopropoxyzirconium, tetrabutoxy zirconium, tetrakis(methylamino) zirconium,tetrakis(ethylamino) zirconium, and mixtures thereof. The metal oxideprecursors are supplied by Sigma-Aldrich.

Non-limiting examples of reagents are water, oxygen, ozone, and mixturethereof.

The metal oxide layer may have thickness of from about 0.4 nm to about 2nm, or from about 0.5 nm to about 1 nm, or from about 0.5 nm to about0.8 nm.

Non-limited examples of a second functional group of the silane layerthat may provide a charge to the core-shell particle or that may modifya surface feature of the core-shell particles are alkyl group, ahalogenated alkyl group, an alkenyl group, an aryl group, a hydroxygroup, a carboxy group, a sulfate group, a sulfonate group, a phosphategroup, a phoshonic group, an amine group, a quaternary ammonium group, adimethylsiloxane group, an ester group, an amide group, and ethyleniminegroup.

Non-limiting examples of first functional groups of the silane compoundthat may react with the metal oxide layer are alkoxy, alkylamino,halide, hydrogen, and hydroxy. This means that a silicon atom of thesilane may be connected to an alkoxy group, an alkylamino group, ahalide group, a hydrogen group (Si—H), and a hydroxy group,respectively.

An example of a class a silane compound for bonding to the metal oxideof the metal oxide layer is trialkoxy silane coupling groups, such as3-(trimethoxysilyl)propyl methacrylate, which is available commerciallyfrom Dow Chemical Company, Wilmington, Del. under the trade name Z6030.The corresponding acrylate may also be used.

According to second embodiment of the present invention, anelectrophoretic medium comprises a core-shell particles and a non-polarfluid, wherein the core comprises an organic pigment, and the shellcomprises a metal oxide layer, a silane layer, and a polymer stabilizerlayer. The metal oxide layer is formed on the surface of the organicpigment using a fluidized bed reactor. The silane compound that is usedto form the silane layer may comprise a first and a third functionalgroups. The first functional group can react with the metal oxide layerto form the silane layer. This silane layer may then be reacted via thethird functional group with a monomer or a macromonomer comprising afourth functional group to form a polymer stabilizer layer.

FIG. 1C illustrates this series of reactions towards the formation ofcore-shell particles 116, wherein the core comprises an organic pigmentand the shell comprises a metal oxide layer, a silane layer, and apolymer stabilizer layer. More specifically, the particle 102 comprisinga core (organic pigment 101) and a metal oxide layer with thickness ofL1, may react with a silane compound 113, comprising a substituent F1having a first functional group and a substituent F3 having a thirdfunctional group. The first functional group may react with the metaloxide layer of particle 102 to form a core-shell particle 114. This coreof core-shell particle comprises an organic pigment, and the shellcomprises a metal oxide layer with thickness L1 and a silane layer withthickness L2. Particle 114 may react via a third functional group insubstituent F2 with a monomer (or macromonomer M1-F4 to form acore-shell particle 115 comprising a shell having a metal oxide layerwith thickness L1, a silane layer with thickness L2, and a polymerstabilizer layer with thickness L3. The polymer stabilizer layer isformed by the reaction of the third functional group in the silanesubstituent F3 and the fourth functional group F4 of the monomer ormacromonomer M1-F4. Silane compound 113 also comprises substituents R4and R5. One or both of substituents R4 and R5 may also comprise a firstfunctional group, which is able to react with the metal oxide layer. Inaddition, one or both substituents R4 and R5 may also comprise a thirdfunctional group, which is able to react with the fourth functionalgroup of the monomer or macromonomer M1-F4.

According to the second embodiment of the present invention, the polymerstabilizer layer may be formed from the reaction of one or more monomersor macromonomers having a fourth functional group and the polymerizablethird functional group of the silane. Various polymerization techniquesknown by those skilled in the art may be applied, such as random graftpolymerization (RGP), ionic random graft polymerization (IRGP), and atomtransfer radical polymerization (ATRP), as described in U.S. Pat. No.6,822,782, the contents of which are incorporated herein by reference inits entirety. As used herein throughout the specification and theclaims, macromonomer means a macromolecule with one end-group thatenables it to act as a monomer.

Suitable monomers for forming the polymer stabilizer layer may include,but are not limited to, styrene, α-methyl styrene, methyl acrylate,methyl methacrylate, n-butyl acrylate, n-butyl methacrylate, t-butylacrylate, t-butyl methacrylate, vinyl pyridine, n-vinyl pyrrolidone,2-hydoxyethyl acrylate, 2-hydroxyethyl methacrylate, dimethylaminoethylmethacrylate, lauryl acrylate, lauryl methacrylate, 2-ethylhexylacrylate, 2-ethylhexyl methacrylate, hexyl acrylate, hexyl methacrylate,n-octyl acrylate, n-octyl methacrylate, n-octadecyl acrylate,n-octadecyl methacrylate, 2-perfluorobutylethyl acrylate, 2,2,2trifluoroethyl methacrylate, 2,2,3,3 tetrafluoropropyl methacrylate,1,1,1,3,3,3-hexafluoroisopropyl acrylate,1,1,1,3,3,3-hexafluoroisopropyl methacrylate,2,2,3,3,3-pentafluoropropyl acrylate, 2,2,3,3-tetrafluoropropylacrylate, 2,2,3,4,4,4-hexafluorobutyl methacrylate, and2,2,3,3,4,4,4-heptafluorobutyl methacrylate or the like. Themacromonomer may contain a terminal functional group selected from thegroup consisting of an acrylate group, a vinyl group, or combinationsthereof

In one embodiment of the present invention, macromonomers orpolymerizable monomers are attached to the surface of the particle toform the polymer stabilizer layer via a reaction with the thirdfunctional group of the silane layer. The third functional group may beepoxy, vinyl, styrene, acryloyl, methacryloyl, methacryloxyakyl, amino,hydroxy, carboxy, alkoxy group, and chloride. An example of a silanecompound that may form the silane layer of the core-shell particle is3-(trimethyoxysilyl)propyl methacrylate, which is available commerciallyfrom Dow Chemical Company, Wilmington, Del. under the trade name Z6030.The corresponding acrylate may also be used.

Other macromonomer and silane compounds that may be used to form thecore-shell particles are described in U.S. Patent Application No.2018/0210312, the contents of which are incorporated herein by referencein its entirety.

One type of macromonomer that may be used to form a polymer stabilizerlayer may be acrylate terminated polysiloxane, such as Gelest, MCR-M11,MCR-M17, or MCR-M22, for example. Another type of macromonomers which issuitable for the process is PE-PEO macromonomers, as shown below:

R_(m)O—[—CH₂CH₂O—]_(n)—CH₂-phenyl-CH═CH₂; or

R_(m)O—[—CH₂CH₂O—]_(n)—C(═O)—C(CH₃)═CH₂.

The substituent R may be a polyethylene chain, n is 1-60 and m is 1-500.The synthesis of these compounds may be found in Dongri Chao et al.,Polymer Journal, Vol. 23, no. 9, 1045 (1991) and Koichi Ito et al,Macromolecules, 1991, 24, 2348. A further type of suitable macromonomersis PE macromonomers, as shown below:

CH₃—[—CH₂—]_(n)—CH₂O—C(═O)—C(CH₃)═CH₂.

The n, in this case, is 30-100. The synthesis of this type ofmacromonomers may be found in Seigou Kawaguchi et al, Designed Monomersand Polymers, 2000, 3, 263.

When choosing a bifunctional compound, such as a silane comprising afirst and a third functional group, to provide polymerizable orinitiating functionality on the particle, attention should be paid tothe relative positions of the two groups within the reagent. As shouldbe apparent to those skilled in polymer manufacture, the rate ofreaction of a polymerizable or initiating group bonded to a particle mayvary greatly depending upon whether the group is held rigidly close tothe particle surface, or whether the group is spaced (on an atomicscale) from that surface and can thus extend into a reaction mediumsurrounding the particle, this being a much more favorable environmentfor chemical reaction of the group. In general, it is preferred thatthere be at least three atoms in the direct chain between the twofunctional groups; for example, the aforementioned3-(trimethoxysilyl)propyl methacrylate provides a chain of four carbonand one oxygen atoms between the silyl and ethylenically unsaturatedgroups, while the aforementioned 4-vinylaniline separates the aminogroup (or the diazonium group, in the actual reactive form) from thevinyl group by the full width of a benzene ring, equivalent to about thelength of a three-carbon chain.

In any of the processes described above, the quantities of the reagentsused (e.g., the organic core pigment particles, the metal oxide layermaterial and the material for forming the polymer stabilizer layers) maybe adjusted and controlled to achieve the desired organic content in theresulting core-shell particles. Furthermore, the processes of thepresent invention may include more than one stage and/or more than onetype of polymerization.

As noted above, the particles made according to the various embodimentsof the present invention are dispersed in an encapsulation fluid. It isdesirable that the polymer stabilizer layer be highly compatible withthe encapsulated fluid. In practice, the suspending fluid in anelectrophoretic medium is normally hydrocarbon-based, although the fluidcan include a proportion of halocarbon, which is used to increase thedensity of the fluid and thus to decrease the difference between thedensity of the fluid and that of the particles. Accordingly, it isimportant that the polymer stabilizer layer formed in the presentprocesses be highly compatible with the encapsulated fluid, and thusthat the polymer stabilizer layer itself comprise a major proportion ofhydrocarbon chains; except for groups provided for charging purposes, asdiscussed below, large numbers of strongly ionic groups are undesirablesince they render the material of the polymer stabilizer layer lesssoluble in the hydrocarbon suspending fluid and thus adversely affectthe stability of the particle dispersion. Also, as already discussed, atleast when the medium in which the particles are to be used comprises analiphatic hydrocarbon suspending fluid (as is commonly the case), it isadvantageous for the material of polymer stabilizer layer to have abranched or “comb” structure, with a main chain and a plurality of sidechains extending away from the main chain. Each of these side chainsshould have at least about four, and preferably at least about six,carbon atoms. Substantially longer side chains may be advantageous; forexample, some of the preferred materials of the polymer stabilizer layermay have lauryl (C₁₂) side chains. The side chains may themselves bebranched; for example, each side chain could be a branched alkyl group,such as a 2-ethylhexyl group. It is believed (although the invention isin no way limited by this belief) that, because of the high affinity ofhydrocarbon chains for the hydrocarbon-based suspending fluid, thebranches of the material of the polymer stabilizer layers spread outfrom one another in a brush or tree-like structure through a largevolume of liquid, thus increasing the affinity of the particle for thesuspending fluid and the stability of the particle dispersion.

There are two basic approaches to forming such a comb polymer. The firstapproach uses monomers, which inherently provide the necessary sidechains. Typically, such a monomer has a single polymerizable group atone end of a long chain (at least four, and preferably at least six,carbon atoms). Monomers of this type, which have been found to give goodresults in the present processes, include hexyl acrylate, 2-ethylhexylacrylate and lauryl methacrylate. Isobutyl methacrylate and2,2,3,4,4,4-hexafluorobutyl acrylate have also been used successfully.In some cases, it may be desirable to limit the number of side chainsformed in such processes, and this can be achieved by using a mixture ofmonomers (for example, a mixture of lauryl methacrylate and methylmethacrylate) to form a random copolymer in which only some of therepeating units bear long side chains. In the second approach, typifiedby an RGP-ATRP process, a first polymerization reaction is carried outusing a mixture of monomers, at least one of these monomers bearing aninitiating group, thus producing a first polymer containing suchinitiating groups. The product of this first polymerization reaction isthen subjected to a second polymerization, typically under differentconditions from the first polymerization, to cause the initiating groupswithin the polymer to cause polymerization of additional monomer on tothe original polymer, thereby forming the desired side chains. As withthe bifunctional reagents discussed above, we do not exclude thepossibility that some chemical modification of the initiating groups maybe effected between the two polymerizations. In such a process, the sidechains themselves do not need to be heavily branched and can be formedfrom a small monomer, for example methyl methacrylate.

Free radical polymerization of ethylenic or similar radicalpolymerizable groups attached to particles may be effected at elevatedreaction temperatures, preferably 60 to 70° C., using conventional freeradical initiators, such as azobis(isobutyryinitrile) (AIBN), while ATRPpolymerization can be effected using the conventional metal complexes,as described in Wang, J. S., et al., Macromolecules 1995, 23, 7901, andJ. Am. Chem. Soc. 1995, 117, 5614, and in Beers, K. et al.,Macromolecules 1999, 32, 5772-5776. See also U.S. Pat. Nos. 5,763,548;5,789,487; 5,807,937; 5,945,491; 4,986,015; 6,069,205; 6,071,980;6,111,022; 6,121,371; 6,124,411; 6,137,012; 6,153,705; 6,162,882;6,191,225; and 6,197,883. The entire disclosures of these papers andpatents are herein incorporated by reference. The presently preferredcatalyst for carrying out ATRP is cuprous chloride in the presence ofbipyridyl (Bpy).

RGP processes of the invention in which particles bearing polymerizablegroups are reacted with a monomer in the presence of an initiator willinevitably cause some formation of “free” polymer not attached to aparticle, as the monomer in the reaction mixture is polymerized. Theunattached polymer may be removed by repeated washings of the particleswith a solvent (typically a hydrocarbon) in which the unattached polymeris soluble, or (at least in the case of metal oxide or other denseparticles) by centrifuging off the treated particles from the reactionmixture (with or without the previous addition of a solvent or diluent),redispersing the particles in fresh solvent, and repeating these stepsuntil the proportion of unattached polymer has been reduced to anacceptable level. (The decline in the proportion of unattached polymercan be followed by thermogravimetric analysis of samples of thepolymer.) Empirically, it does not appear that the presence of a smallproportion of unattached polymer, of the order of 1 percent by weight,has any serious deleterious effect on the electrophoretic properties ofthe treated particles; indeed, in some cases, depending upon thechemical natures of the unattached polymer and the suspending fluid, itmay not be necessary to separate the particles having attached a polymerstabilizer layer from the unattached polymer before using the particlesin an electrophoretic display.

It has been found that there is an optimum range for the amount ofpolymer stabilizer layer which should be formed on electrophoreticparticles, and that forming an excessive amount of polymer on theparticles can degrade their electrophoretic characteristics. The optimumrange will vary with a number of factors, including the density and sizeof the particles being coated, the nature of the suspending medium inwhich the particles are intended to be used, and the nature of polymerformed on the particles, and for any specific particle, polymer andsuspending medium, the optimum range is best determined empirically.However, by way of general guidance, it should be noted that the denserthe particle, the lower the optimum proportion of polymer by weight ofthe particle, and the more finely divided the particle, the higher theoptimum proportion of polymer. In general, the particles should becoated with at least about 2, and desirably at least about 4, percent byweight of the particle. In most cases, the optimum proportion of polymerwill range from about 4 to about 15 percent by weight of the particle,and typically is about 6 to about 15 percent by weight, and mostdesirably about 8 to about 12 percent by weight.

To incorporate functional groups for charge generation of the pigmentparticles, a co-monomer may be added to the polymerization reactionmedium. The co-monomer may either directly charge the core-shellparticles or interact with a charge control agent in the display fluidto bring a desired charge polarity and charge density to the core-shellparticles. Suitable co-monomers may include vinylbenzylaminoethylamino-propyl-trimethoxysilane,methacryloxypropyltrimethoxysilane, acrylic acid, methacrylic acid,vinyl phosphoric acid, 2-acrylamino-2-methylpropane sulfonic acid,2-(dimethylamino)ethyl methacrylate,N-[3-(dimethylamino)propyl]methacryl amide and the like. Suitableco-monomers may also include fluorinated acrylate or methacrylate suchas 2-perfluorobutylethyl acrylate, 2,2,2 trifluoroethyl methacrylate,2,2,3,3 tetrafluoropropyl methacrylate, 1,1,1,3,3,3-hexafluoroisopropylacrylate, 1,1,1,3,3,3-hexafluoroisopropyl methacrylate,2,2,3,3,3-pentafluoropropyl acrylate, 2,2,3,3-tetrafluoropropylacrylate, 2,2,3,4,4,4-hexafluorobutyl methacrylate or2,2,3,3,4,4,4-heptafluorobutyl methacrylate. Alternatively, charged orchargeable groups may be incorporated into the polymer via thebifunctional stabilizer used to provide polymerizable or initiatingfunctionality to the pigment.

Functional groups, such as acidic or basic groups, may be provided in a“blocked” form during polymerization, and may then be de-blocked afterformation of the polymer. For example, since ATRP cannot be initiated inthe presence of acid, if it is desired to provide acidic groups withinthe polymer, esters such as t-butyl acrylate or isobornyl methacrylatemay be used and the residues of these monomers within the final polymerhydrolyzed to provide acrylic or methacrylic acid residues.

When it is desired to produce charged or chargeable groups on thepigment particles and also polymer stabilizer layers separately attachedto the particles, it may be very convenient to treat the particles(after the metal oxide coating) with a mixture of two reagents, one ofwhich carries the charged or chargeable group (or a group which willeventually be treated to produce the desired charged or chargeablegroup), and the other of which carries the polymerizable orpolymerization-initiating group. Desirably, the two reagents have thesame, or essentially the same, functional group which reacts with theparticle surface so that, if minor variations in reaction conditionsoccur, the relative rates at which the reagents react with the particleswill change in a similar manner, and the ratio between the number ofcharged or chargeable groups and the number of polymerizable orpolymerization-initiating groups will remain substantially constant. Itwill be appreciated that this ratio can be varied and controlled byvarying the relative molar amounts of the two (or more) reagents used inthe mixture. Examples of reagents which provide chargeable sites but notpolymerizable or polymerization-initiating groups include3-(trimethoxysilyl)propylamine,N[3-(trimethoxysilyl)propyl]diethylenetriamine,N[3-(trimethoxysilyl)propyl]ethylene and 1-[3-(trimethoxysilyl)propyl]urea; all these silane reagents may be purchased from UnitedChemical Technologies, Inc., Bristol, Pa., 19007. As already mentioned,an example of a reagent, which provides polymerizable groups but notcharged or chargeable groups, is 3-(trimethoxysilyl)propyl methacrylate.

The core-shell particles of the present invention are useful in theelectrophoretic field. Firstly, the shell of the particles enables themodification and control of the surface nature and charge of the organicpigment particle. Thus, different types of electrophoretic particles ina medium may be surface modified using different silane treatment, whichmay contribute to an effective separation and, as a result, an improvedelectro-optic performance. The metal oxide layer enables the covalentlyattachment of the silane layer onto the particle surface. Contrary toinorganic pigments, which may comprise functional groups, such ashydroxyl, that provide a reactive anchor for an organic species to beattached on their surface, most organic pigments do not containfunctional groups that can be readily reacted with functional groups oftypical reagents. Thus, by precipitating a layer of a metal oxide ontoan organic pigment particle surface, the subsequent silane layer isstrongly attached on the particle's surface and it is unlikely that itwill be desorbed from the surface, increasing the effectiveness of thetreatment. The same is true for surface treatments that include apolymer stabilizer layer. This layer contributes to the stability ofparticle dispersion because it protects against particle aggregation.Steric effects, caused by the polymer attachment on the pigment particlesurfaces, prevent the particles from aggregating. The stronger theattachment, the more effective the stabilization is, because lessdesorption of the polymer from the particle surface is observed withstronger attachment. Thus, in the case of a covalent bond of the polymerto the particle surface, a more effective particle stabilization andimproved electro-optic performance is typically observed.

The process by which the shell of the core-shell particle is formed alsocontributes to improved electro-optic performance. Typically, a metaloxide layer is formed by the precipitation of a metal oxide created bythe reaction of a metal oxide precursor with a reagent in an organicsolvent. The generated metal oxide precipitates on the surface of thepigment particle, which is present in the solvent. The liquid phaseprocess may result in a non-uniform coating of metal oxide, which meansthat for a more complete surface coverage, a larger amount of metaloxide is required. On the contrary, the process disclosed herein using afluidized bed reactor, a gaseous stream of a metal oxide precursor, andthen a gaseous stream of a reagent, enables the potential formation of auniform metal oxide layer that has a lower thickness. This translates toa better optical performance of the core-shell particle, because thickermetal oxide layer may lead to higher light reflection on the shell,which would prevent optimum light absorption by the organic pigmentparticle and appearance of less saturated color. The gaseous stream ofthe metal oxide precursor in an inert gas enables the complexation (oradsorption) of the metal oxide precursor on the surface of the organicpigment particle. The process does not favor the presence of a largeexcess of the precursor on the organic pigment particle, which leads toa more uniform and thin metal oxide layer.

According to one aspect of the present invention, organic pigmentparticles comprising a metal oxide layer may be manufactured using afluidized bed reactor as described above. The organic pigment particlesare inserted into the reactor as a powder bed and contacted with agaseous stream comprising a precursor of metal oxide and an inert gas toform a mixture of organic pigment particles and metal precursor. Themetal oxide precursor may be complexed on the organic pigment particlesurface and then reacted with the reagent, providing a metal oxidecoating on the surface of the organic pigment particles. The method maytake place in a continuous or a batch process.

The amount of polymer stabilizer layer on the core-shell particles maybe controlled. Forming an excessive amount of polymer on the particlescan degrade their electrophoretic characteristics. The optimum rangewill vary with a number of factors, including the density and size ofthe organic pigment, the density and thickness of the metal oxide layer,the nature of the non-polar fluid of the electrophoretic medium, and thenature of material of the polymer stabilizer layer. It was found thatthe denser the particle, the lower the optimum proportion of polymerstabilizer layer by weight of the core-shell particle. In addition, themore finely divided the organic pigment core, the higher the optimumproportion of polymer stabilizer layer. The polymer stabilizer layer maybe from 1 to 50 weight percent, or from 2 to 30 weight percent, or from4 to 20, or from 5 to 15 weight percent by weight of the core-shellparticle.

The electrophoretic medium of the present invention that comprises thecore-shell particles can be used to form an electrophoretic device. Theelectrophoretic device may be an electrophoretic display, comprising anelectro-optic material layer comprising the electrophoretic medium,wherein the electrophoretic medium may be encapsulated in microcapsulesor microcells. An example of such an electrophoretic device 200 isillustrated in FIG. 2. In this example, the electrophoretic devicecomprises an electro-optic material layer 225, comprisingelectrophoretic medium 220, which is encapsulated in microcapsules 250.The electrophoretic device also comprises a first light transmissiveelectrode layer 210 and a second electrode layer 240. The secondelectrode layer 240 is adhered to the electro-optic material layer by anadhesive layer 230. The electrophoretic device may comprise a secondadhesive layer (not shown in FIG. 2), which is used to adhere the firstlight-transmissive layer 210 to the electro-optic material layer 225.The electro-optic material layer 225 may comprise, in addition tomicrocapsules 250, a binder 222. In the example of FIG. 2, theelectrophoretic medium 220 comprises two types of particles in anon-polar fluid. One or more of the types of particles may be core-shellcomprising an organic pigment core, and a shell comprising a metal oxidelayer and a silane layer. The particles that can be caused to move withthe application of an electric field across the microcapsules 250.

The electrophoretic medium of the present invention can be used to formelectrophoretic assemblies, such as a front plane laminate and a doublerelease sheet. As illustrated in FIG. 3, in some embodiments, a frontplane laminate 300 comprises a light-transmissive electrode layer 310,an electro-optic material layer 325, and a release sheet 360. Therelease sheet 360 is adhered to the electro-optic material layer 325 byan adhesive layer 330. The electro-optic material layer 325 maycomprise, in addition to microcapsules 350, a binder 322. In the exampleof FIG. 3, the electrophoretic medium 320 comprises two types ofparticles in a non-polar fluid. One or more of the types of particlesmay be core-shell comprising an organic pigment core, and a shellcomprising a metal oxide layer and a silane layer. Removal of therelease sheet 360 and connecting a backplane, comprising an electrodelayer, onto the exposed surface of the electro-optic material layer 325via the adhesive layer 330, results in the formation of anelectrophoretic device.

In another embodiment, as illustrated in FIG. 4, a double release sheet400 comprises two adhesive layers (475 and 485) and two release sheets(470 and 480). Specifically, in this example, a first release sheet 470is attached to the electro-optic material layer 425 using a firstadhesive layer 475. A second release sheet 480 is attached to theelectro-optic material layer 425 using a second adhesive layer 485. Inthe example of FIG. 4, the electrophoretic medium 420 comprises twotypes of particles in a non-polar fluid. One or more of the types ofparticles may be core-shell comprising an organic pigment core, and ashell comprising a metal oxide layer and a silane layer. Removal of therelease sheet 470 and connecting a first light-transmissive electrodelayer onto the exposed surface of the electro-optic material layer 425via the adhesive layer 475, and removal of the release sheet 480 andconnecting a backplane, comprising a second electrode, onto the exposedsurface of the other side of the electro-optic material layer 425,results in the formation of an electrophoretic device.

In another embodiment, the electrophoretic medium of the presentinvention can be used to form an electro-optic assembly, wherein theelectro-optic assembly is an inverted front plane laminate. The invertedfront plane laminate comprises in order (i) a first electrode layer,(ii) a first adhesive layer, (iii) an electro-optic material layercomprising an encapsulated electrophoretic medium, and (iv) and arelease sheet. The inverted front plane laminate may also comprise asecond adhesive layer between the electro-optic material layer and theelectro-optic material layer. The inverted front plane laminate can beconverted to an electro-optic device by removing the release sheet andconnecting a second electrode layer onto the exposed electro-opticmaterial layer (or onto the second adhesive lay.

The electrophoretic medium of the electrophoretic device or theelectrophoretic assemblies of the present inventions comprise aplurality of at least one type of the disclosed core-shell particles.The electrophoretic medium may further comprise a plurality of anothertype of core-shell particles or a different type of chargedelectrophoretic particles. The electrophoretic medium may also comprisemore than two types of core-shell particles or other types of chargedelectrophoretic particles. The different types of core-shell particlesmay have different colors.

The non-polar fluid, in which the electrophoretic particles aredispersed, may be clear and colorless. It preferably has a dielectricconstant in the range of about 2 to about 30, preferably about 2 toabout 15 for high particle mobility. Examples of suitable dielectricsolvent include hydrocarbons such as isopar, decahydronaphthalene(DECALIN), 5-ethylidene-2-norbornene, fatty oils, paraffin oil, siliconfluids, aromatic hydrocarbons such as toluene, xylene,phenylxylylethane, dodecylbenzene or alkylnaphthalene, halogenatedsolvents such as perfluorodecalin, perfluorotoluene, perfluoroxylene,dichlorobenzotrifluoride, 3,4,5-trichlorobenzotri fluoride,chloropentafluoro-benzene, dichlorononane or pentachlorobenzene, andperfluorinated solvents such as FC-43, FC-70 or FC-5060 from 3M Company,St. Paul Minn., low molecular weight halogen containing polymers such aspoly(perfluoropropylene oxide) from TCI America, Portland, Oreg.,poly(chlorotrifluoro-ethylene) such as Halocarbon Oils from HalocarbonProduct Corp., River Edge, N.J., perfluoropolyalkylether such as Galdenfrom Ausimont or Krytox Oils and Greases K-Fluid Series from DuPont,Del., polydimethylsiloxane based silicone oil from Dow-corning (DC-200).

The content of the electrophoretic particles in the non-polar fluid mayvary. For example, one type of particles may take up 0.1% to 50%,preferably 0.5% to 15%, by volume of the non-polar fluid.

EXAMPLES

Examples of the present invention is described below. The presentinvention is not limited to the Examples.

Example 1

Pigment Red 122 particles with a metal oxide layer: An amount of 151.2 gof Pigment Red 122 powder (a quinacridone pigment supplied as Ink JetMagenta E 02 by Clariant, Basel, Switzerland) was loaded into afluidized bed reactor which was fed with trimethylaluminum in a nitrogenstream and then with a water/nitrogen stream. The resulting particleswere dried to provide Pigment Red 122 particles having an aluminum oxidelayer of approximately 1 nm in thickness.

Example 2

Pigment Red 122 particles with a metal oxide layer and a silane layer:Into a 100 mL plastic bottle were added (a) 10.0 g of the Pigment Red122 particles with a metal oxide layer from Example 1, (b) 40 g ofethanol, (c) 0.5 g of de-ionized water and (d) 100 g of zirconia beads(1.7-2.4 mm). This mixture was placed on a roll mill and mixed forapproximately 16 hours. Then, the dispersion was filtered through a 200micron mesh into a round bottom flask. This is Sample A.

Into a separate 100 mL plastic bottle were added 7.1 g of a 40 weight %solution of vinylbenzylaminoethylaminopropyltrimethoxysilane inmethanol, (supplied by Dow as Xiameter OFS-6032 silane), 1.6 g ofde-ionized water and 0.6 g of glacial acetic acid. This mixture wasplaced on a roll mill for one hour. This is Sample B.

Sample B was then added into the flask, containing Sample A. The pH ofthe mixture was adjusted to 9 using a 0.1M solution of ammoniumhydroxide. The resulting dispersion was stirred at room temperatureusing an overhead mixer for 60 minutes and 30 g of ethanol were addedinto the flask. The resulting mixture was centrifuged at 5000 rpm for 30minutes. The supernatant liquid was decanted and the remaining materialwas dried overnight at 70° C. using a vacuum oven. This is Sample C.

Example 3

Core-Shell particles with a metal oxide layer, a silane layer, and apolymer stabilizer layer: Into a 250 mL plastic bottle were added (a) 10g of Sample C particles from Example 2, (b) 72.5 g of toluene and (c)13.3 g of lauryl methacrylate (LMA). The dispersion was mixed using anIKA Ultra Turrax mixer at 12,000 rpm for 1 hour. Then, the dispersionwas transferred into a round bottom flask, which was purged withnitrogen and heated to 65° C. After one hour of heating at to 65° C., aninitiator solution comprising 0.2 g of2,2′-azobis(2-methylpropionitrile) (AIBN) in 4.2 g of toluene wasrapidly injected into the flask and the reaction was allowed to proceedfor 16 hours. The resulting dispersion was collected in a 1-L plasticbottle and centrifuged at 4500 rpm for 30 minutes. The supernatantliquid was discarded. The remaining material was mixed with 200 mL oftoluene and the supernatant liquid was discarded. The rinsing withtoluene, the centrifugation and removal of the supernatant liquid wasperformed one more time. Then, the remaining material was collected anddried in a 70° C. vacuum oven overnight. The resulting solid particlesis Sample D.

Example 4

Electrophoretic medium: Into a 100 mL container were added (a) 8.65 g ofparticles from Sample D, (b) 36.9 g of isoparaffin hydrocarbon solvent(Isopar E, supplied by ExxonMobil), and (c) 2.47 g of a 70 wt % solutionof CCA041. This mixture was sonicated for 90 minutes and then mixed in arolled for one hour. The sonication and mixing were repeated 10 times.Then, the dispersion was filtered through a 200 micron mesh.

Example 5

Pigment Red 122 particles with a metal oxide layer and a silane layer:Into a 100 mL plastic bottle were added (a) 10.0 g of the Pigment Red122 particles with a metal oxide layer from Example 1, (b) 40 g ofethanol, (c) 0.5 g of de-ionized water and (d) 100 g of zirconia beads(1.7-2.4 mm). This mixture was placed on a roll mill and mixed forapproximately 16 hours. Then, the dispersion was filtered through a 200micron mesh into a round bottom flask. This is Sample E.

Into a separate 100 mL plastic bottle were added 4.7 g ofmethacryloxypropyl trimethoxysilane, (supplied by Dow as XiameterOFS-6030 silane), 1.6 g of de-ionized water and 0.6 g of glacial aceticacid. This mixture was agitated on a roll mill for one hour. This isSample F.

Sample F was then added into the flask containing Sample E. The pH ofthe mixture was adjusted to 9 using a 0.1M solution of ammoniumhydroxide. The resulting dispersion was stirred at room temperatureusing an overhead mixer for 60 minutes and 30 g of ethanol were addedinto the flask. The resulting mixture was centrifuged at 5000 rpm for 30minutes. The supernatant liquid was decanted and the remaining materialwas dried overnight at 70° C. using a vacuum oven. This is Sample G.

Example 6

Electrophoretic medium: Into a 100 mL container were added (a) 8.65 g ofparticles from Sample G, (b) 36.9 g of isoparaffin hydrocarbon solvent(Isopar E, supplied by ExxonMobil), and (c) 2.47 g of a 70 wt % solutionof CCA041. This mixture was sonicated for 90 minutes and then mixed in arolled for one hour. The sonication and mixing were repeated 10 times.Then, the dispersion was filtered through a 200 micron mesh.

Example 7

Control Sample of Pigment Red 122 (supplied by Clariant, Basel,Switzerland, as Ink Jet Magenta E 02) was modified with vinylbenzylchloride and lauryl methacrylate polymer as described in U.S. PatentApplication 2014/0340430 (Example 1).

Thermogravimetric Analysis: The extent of silane-polymer attachment wasassessed using thermogravimetric analysis (TGA). Over a temperaturerange of 115-365° C., a relative mass loss of 13.6% was observed forSample D of Example 3 and 11.0% for Sample G of Example 5. By contrast,a mass loss of just 3.6% was observed over the same temperature rangefor the unmodified Pigment Red 122 Ink Jet Magenta E 02. These resultsindicate that the silane and polymer are successfully bonded to thepigment surface.

Zeta Potential Measurement: Zeta potential values of the pigment sampleswere measured by titration of the corresponding pigment dispersions inIsopar E using a standard solution of a charge control agent in Isopar Eand a Colloidal Dynamics ZetaProbe.

Results of zeta potential measurements for pigment particles fromExample 3 (Sample D), Example 5 (Sample G) and Example 7 (Control) areprovided in the Graph of FIG. 5. The data of FIG. 5 indicate thatPigment 122 functionalized with aluminumoxide—vinylbenzylaminoethylaminopropyltrimethoxysilane—laurylmethacrylate, prepared in Example 3, has a zeta potential plateau of˜105 mV. Pigment 122 functionalized with aluminumoxide—methacryloxypropyl trimethoxysilane—lauryl methacrylate, preparedin Example 6, has a zeta potential plateau of ˜8 mV. This demonstratesthat zeta potential can be tuned over a broad range for the pigmentcomprising a metal oxide layer with systematic changes to thefunctionalization materials. By contrast, the zeta potential of PigmentRed 122 functionalized with vinylbenzyl chloride-lauryl methacrylate(without a metal oxide layer) has a plateau at ˜50 mV.

While preferred embodiments of the invention have been shown anddescribed herein, it will be understood that such embodiments areprovided by way of example only. Numerous variations, changes, andsubstitutions will occur to those skilled in the art without departingfrom the spirit of the invention. Accordingly, it is intended that theappended claims cover all such variations as fall within the spirit andscope of the invention.

All of the contents of the aforementioned patents and applications areincorporated by reference herein in their entireties.

1. An electrophoretic medium comprising a plurality of a first type ofcore-shell particles and a non-polar fluid, wherein each of theplurality of the first type of core-shell particles comprises: a corecomprising an organic pigment; a shell comprising a metal oxide layerand a silane layer; wherein the thickness of the metal oxide layer isfrom about 0.4 nm to about 2 nm; wherein the silane layer is formed froma silane compound comprising a first functional group, wherein the firstfunctional group reacts with the metal oxide.
 2. An electrophoreticmedium comprising a plurality of a first type of core-shell particlesand a non-polar fluid, wherein each of the plurality of the first typeof the core-shell particles comprises: a core comprising an organicpigment; a shell comprising a metal oxide layer and a silane layer;wherein the metal oxide layer is formed on the surface of the organicpigment using a fluidized bed reactor by inserting the organic pigmentinto the reactor as a powder bed, contacting the powder bed with agaseous stream comprising an inert gas and a metal oxide precursor, andcontacting the powder bed with a gaseous stream of an inert gas and areagent that reacts with the metal oxide precursor to form a metaloxide; and wherein the silane layer is formed from a silane compoundcomprising a first functional group, wherein the first functional groupreacts with the metal oxide.
 3. The electrophoretic medium according toclaim 2, wherein the silane compound further comprises a secondfunctional group, wherein the second functional group is selected fromthe group consisting of an alkyl group, a halogenated alkyl group, analkenyl group, an aryl group, a hydroxy group, a carboxy group, asulfate group, a sulfonate group, a phosphate group, a phoshonic group,an amine group, a quaternary ammonium group, a dimethylsiloxane group,an ester group, an amide group, and ethylenimine group.
 4. Theelectrophoretic medium according to claim 2, wherein the shell furthercomprises a polymer stabilizer layer, wherein the polymer stabilizerlayer is formed from the reaction of the silane layer and a monomer ormacromonomer, wherein the silane compound comprises a third functionalgroup, wherein the monomer or macromonomer comprises a fourth functionalgroup, and wherein the third functional group of the silane compoundreacts with the fourth functional group of the monomer or macromonomer.5. The electrophoretic medium according to claim 2, wherein the metaloxide layer comprises aluminum oxide, silica, titanium dioxide,zirconium oxide, zinc oxide or mixtures thereof.
 6. The electrophoreticmedium according to claim 2, wherein the first functional group isselected from the group consisting of alkoxy, alkylamino, halide,hydrogen, and hydroxy.
 7. The electrophoretic medium according to claim2, wherein the metal oxide precursor is selected from the groupconsisting of trimethylaluminum, triethylaluminum, dimethylaluminumchloride, diethylaluminum chloride, trimethoxyaluminum,triethoxyaluminum, dimethylaluminum propoxide, aluminum triisopopoxide,tributoxy aluminum, tris(dimethylamino) aluminum, tris(diethylamino)aluminum, tris(propylamino) aluminum, aluminum trichloride,trichlorosilane, hexachlorodisilane, silicon tetrachloride,tetramethoxysilane, tetraethoxysilane, tris(tert-pentoxy)silanol,tetraisocyanatesilane, silicon tertrachoride, tris(methylamino)silane,tris(ethylamino)silane, titanium tetrachloride, titanium tetraiodide,tetramethoxy titanium, tetraethoxy titanium, titanium isopropoxide,tetrakis(methylamino) titanium, tetrakis(ethylamino) titanium, dimethylzinc, diethyl zinc, methyl zinc isopropoxide, zirconium tetrachloride,zirconium tetraiodide, tetramethoxy zirconium, tetraethoxy zirconium,tetraisopropoxy zirconium, tetrabutoxy zirconium, tetrakis(methylamino)zirconium, tetrakis(ethylamino) zirconium, and mixtures thereof.
 8. Theelectrophoretic medium according to claim 2, wherein the reagent isselected from the group consisting of water, oxygen, ozone, and mixturethereof.
 9. The electrophoretic medium according to claim 2, wherein themetal oxide layer has thickness of from about 0.5 nm to about 2 nm. 10.The electrophoretic medium according to claim 4, wherein the thirdfunctional group of the silane compound is selected from the groupconsisting of epoxy, vinyl, styrene, acryloyl, methacryloyl,methacryloxyakyl, amino, hydroxy, carboxy, alkoxy group, and chloride.11. The electrophoretic medium according to claim 4, wherein the fourthfunctional group of the monomer or macromonomer is selected from thegroup consisting of vinyl, styrene, acryloyl, methacryloyl,methacryloxyakyl, epoxy, amino, hydroxy, carboxy, and chloride.
 12. Theelectrophoretic medium according to claim 2, wherein the organic pigmentis selected from the group consisting of an azo pigment, aphthalocyanine pigment, a quinacridone pigment, a perylene pigment, adiketopyrrolopyrrole pigment, a benzimidazolone pigment, an isoindolinepigment, an anthranone pigment, an indanthrone pigment, a carbon blackpigment, a rhodamine pigment, a benzinamine pigment, a carbon blackpigments, and mixtures therein.
 13. The electrophoretic medium accordingto claim 2, wherein the organic pigment is selected from the groupconsisting of C.I. Pigment Blue 15, 15:1, 15:2, 15:3, 15:4 15:6, 60, and79; Pigment Red 2, 4, 5, 9, 12, 14, 38, 48:2, 48:3, 48:4, 52:2, 53:1,57:1, 81, 112, 122, 144, 146, 147, 149, 168, 170, 176, 177, 179, 184,185, 187, 188, 208, 209, 210, 214, 242, 254, 255, 257, 262, 264, 282,and 285; C.I. Pigment Violet 1, 19, 23, and 32; C.I. Pigment Yellow 1,3, 12, 13, 14, 15, 16, 17, 73, 74, 81, 83, 97, 109, 110, 111, 120, 126,127, 137, 138, 139, 150, 151, 154, 155, 174, 175, 176, 180, 181, 184,191, 194, 213 and 214; C.I. Pigment Green 7, and 36; C.I. Pigment Black1, and 7; C.I. Pigment Brown 25, 32, 41; Pigment Orange 5, 13, 34, 36,38, 43, 61, 62, 64, 68, 67, 72, 73, and 74, and mixtures thereof.
 14. Anelectrophoretic device comprising a first light-transmissive electrodelayer; an electro-optic material layer comprising encapsulatedelectrophoretic medium; and a second electrode layer; wherein theelectrophoretic medium is the electrophoretic medium of claim
 2. 15. Anelectrophoretic assembly comprising in order: a first light-transmissiveelectrode layer; an electro-optic material layer comprising encapsulatedelectrophoretic medium; an adhesive layer; and a release sheet; whereinthe electrophoretic medium is the electrophoretic medium of claim
 2. 16.An electrophoretic assembly comprising in order: a first release sheet;a first adhesive layer; an electro-optic material layer comprisingencapsulated electrophoretic medium; an adhesive layer; a secondadhesive layer; and a second release sheet; wherein the electrophoreticmedium is the electrophoretic medium of claim
 2. 17. A method ofmanufacturing of an electrophoretic medium comprising a plurality ofcore-shell particle and a non-polar fluid comprising the steps of:providing organic pigment particles; introducing the organic pigmentparticles into a fluidized bed reactor as a powder bed; contacting thepowder bed with a gaseous stream comprising an inert gas and a precursorof metal oxide; contacting the powder bed with a gaseous streamcomprising a reagent to form organic pigment particles having a metaloxide layer on its surface, wherein the reagent is selected from thegroup consisting of water, oxygen, ozone, and mixtures thereof; reactingthe organic pigment particles having a metal oxide layer with a silanecompound in an organic solvent to form a silane layer, wherein thesilane compound comprises a first functional group and a thirdfunctional group, wherein the first functional group reacts with themetal oxide to form organic pigment particles comprising a metal oxidelayer and a silane layer; combining the plurality of core-shellparticles and the non-polar fluid.
 18. The method of manufacturing of anelectrophoretic medium of claim 7 further comprising, before the step ofcombining the plurality of core-shell particles and non-polar fluid, astep of reacting the organic pigment particles having a metal oxidelayer and a silane layer with a monomer or a macromonomer comprising afourth functional group to form a plurality of core-shell particles,wherein the third functional group of the silane reacts with the fourthfunctional group of the monomer or macromonomer.